What Is ClamXav and do Mac users really need antivirus?
Short answer: No.
Longer answer: As the number of viruses for OS X is none, no, they don't. What part of "none" don't these people understand?
There are some 55 viruses for the Mac OS (now called "Classic" by Apple "Dark Age" would be more appropriate) if that worries you and you haven't removed it yet, follow the directions in the NSA's guide:
Oh, and turn off macros in MS Office, if you're running that.
Now, you have NO malware to guard against. So why run a, hence useless, AV program? In order to slow the system down, possibly screw it up (yes, it's happened) and line the pockets of the likes of Symantec. Give me a break.